1. Field of the Invention:
The present invention relates to an electric element for electric resistance furnaces and more particularly to an electric heating element for electric heating furnaces which is used in various heat treatments, such as melting of metal, glazing of ceramic wares or the like.
2. Description of the Prior Art:
An electric resistance furnace has been generally widely used in heat treatment, melting of various samples, glazing of ceramic wares, and particularly in precise or continuous treatment processes because the furnace using an electric heater as its heat source can obtain respectively high temperature and temperature control and handling is easy.
In the conventional electric resistance furnace, as shown in FIG. 1, a heating chamber is enclosed by walls having a layered structure from the exterior as follows: a metal board, adiabatic layers for keeping warm, and ceramic bricks on which channel shaped heater receiving portions are provided. A heater element of coil resistance wire having circular shape in section or corrugated resistance wire having rectangular shape in section is received and maintained in the heat receiving portions to prevent deformation by heat of the resistance wire at high temperature.
However, in the heater element so supported, the greater part (about 1/2-1/3) of the heat generating surface is covered by the ceramic bricks and therefore heat from the heater forms an atmosphere of high temperature in the heating chamber after rising the temperature of the ceramic bricks at the heater receiving portions. Because of this, the heat capacity of the entire furnace is remarkably large thereby to necessitate useless treating time and electric power dissipation (electric heats). Accordingly, such a furnace cannot function to rapidly rise or lower the temperature therein. It is difficult to form heater receiving portions having complicated configuration on the fragile ceramic bricks and the manufacturing cost of such a furnace is high. Further, owing to provision of the ceramic bricks, there are defects in that the furnace is heavy, difficult to carry and install and large in size. In addition, because the ceramic bricks are high in moisture absorption, there are defects in that oxygen gas is generated by contained moisture to be separated at high temperature so as to oxidize the samples and in that leak loss occur. Particularly, because dry burning is necessitated at high moisture conditions, it has been greatly desired to keep the shape of the heating element stable under the high temperature without using the ceramic bricks for keeping shape stable.
As shown in FIG. 2, there has been provided a resistance wire having circular shape in section coiled in a spiral cylinder along a heating chamber, and ceramic supports are inserted between spiral pitches to keep shape of the resistance wire at an atmosphere of high temperature. However, since the thickness of such resistance wire is limited with respect to the resistance value and may be deformed under the influence of heat if it is not cylindrical, there is a defect in that the furnace used is limited to the cylindrical shape. Also, because the spaces between pitches are easy to vary, and since a number of ceramic supports must be used, there are the same defects as that of the furnace shown in FIG. 1, for example, impossible rapid rising or lowering of temperature, expensive cost of manufacture, generation of oxygen gas or the like. Further, removing and mounting of the resistance wire in case of a defect, such as disconnection, deterioration or the like, destroys the ceramic bricks, and also the resistance wire shown in FIG. 2 must embed a lead wire passing through a power source terminal into an inner wall (usually ceramics) of the heating chamber. Accordingly, change of the electric heating element is not easy in the structures mentioned above.
As described above, the conventional heater wire having circular shape in section necessitates ceramic supports in order to keep the shape of the electric heating element in the atmosphere of high temperature and therefore it has been impossible to prevent oxygen gas being generated in the heating chamber in order to heat treat without oxidation the sample because an electric heating element shape keeping material which is suitable does not exist besides the known ceramics.
Further, it has not been proposed that the electric heating element be of a cartridge type to be capable of easily being exchanged.
Although it has been proposed to use a surface resistance element of mesh type as an electric heating element, because it is difficult to adjust temperature, such a surface resistance element is used only in a heat treatment furnace operated at lower temperature and can not be used for a high temperature furnace. Therefore, it is desirable to maintain a wire resistance body at an individual condition under a high temperature without using the shape keeping material.